The present invention relates to pressure responsive switches, and more particularly pertains to a combination differential and static pressure switch for determining a minimum static pressure within a test system, and also for detecting when a differential pressure condition occurs within the test system.
For a variety of industrial purposes the working pressure of pipe, such as drill pipe, gas pipe, plastic pipe, and water pipe, etc., must be accurately measured and tested to determine their reliability and structural integrity during their working life. In addition, the standard working pressure of plastic containers and glass bottles must also be measured and determined in order to determine their reliability and structural integrity during their working life. In fact, it can be broadly stated that any item of manufacture in commerce that has the possibility of breaking, rupturing, and splitting, thus causing a leak, can be measured to determine their standard working pressure and psi. rating.
A prominent method of measuring and determining the standard working pressure of a workpiece, such as the above-mentioned items, is by a time-to-failure test. The workpiece is held at a constant pressure at a predetermined psi. for a given time period so that data points can be collected. What is being measured is hoop stress: the stress generated in the workpiece by pressure being exerted thereon and occurring along the circumference of the workpiece during the testing. Hoop stress is the force that attempts to tear apart and split the pressurized pipe or tubing when the pipe is subjected to pressure.
As an example of a time-to-failure test, if seven pieces of industrial pipe are held at 600 psi., they may fail in ten hours; if six pieces of industrial pipe are held at 400 psi., they may fail in one hundred hours; and, if six more pieces of industrial pipe are held at 300 psi., they may fail after one thousand hours. By calculating out this curve, it can be determined that the industrial pipe being tested at a working pressure of 200 psi. will last fifty years. Therefore, if the working pressure is limited to 200 psi. based on the above curve calculation, the industrial pipe will last fifty years. The industrial pipe can be rated at 200 psi. and sold as a 200 psi. pipe. Furthermore, data points are collected along the curve which is developed during pipe testing, and then a line is drawn through all the data points; this represents a statistical analysis of the failure points. By determining how wide the data point spread is, confidence limits can be ascertained for the particular industrial pipe being tested at a predetermined psi. for a given time period. Essentially, the pipe is being tested for a leak, and some tests are timed to run for years.
The above-described test is not a production test; it is an experimental test. The test is undertaken to determine when the workpiece fails. The test continues until the workpiece fails --the workpiece develops a detectable leak while being held at a predetermined constant pressure. Also, it should be noted that this is destructive testing: the workpiece to be tested is chopped off, tested, and then discarded after the testing is completed. The differential pressure switches used in such testing are only one component of a larger test system which includes metering valves, a pumping unit, regulators, pressure gauges, control panels, and a visual display in the form of LED lights or a computer display screen.
While the prior art discloses a number of pressure switches used in pressure testing, there is a need for a differential pressure switch which detects both a differential and the static pressure condition and also has an extremely high degree of repeatability and a high degree of accuracy. In addition, there remains a need for a differential pressure switch which is responsive to a high pressure and a low differential as well as a low pressure and a low differential.